Removes Isolate from compilation info and instead threads isolate through
function calls. This ensures that we can't access the isolate from
background thread compilations.

BUG=v8:5203

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I9a4e1cd67c4736e36f609360b996fb55166a1c50
Reviewed-on: https://chromium-review.googlesource.com/751745
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49386}